This invention relates to manufacturing coated optical fibers.
Optical fibers typically are silica-based. To improve the moisture resistance and mechanical properties of the fiber, the fiber is often provided with multiple polymeric coatings disposed concentrically about the fiber, with the coating nearest the fiber being more flexible than the outermost coating(s).
To form the coatings, a photopolymerizable composition typically is applied to the fiber and polymerized by exposure to actinic radiation, e.g., ultraviolet radiation, to form a first polymer coating. Next, a second photopolymerizable composition is applied to the first polymer coating and likewise exposed to actinic radiation to form a second polymer coating.
One problem encountered with such coated fibers is that both polymerization processes generate heat. The heat generated during the second polymerization process can lead to the development of tensile stresses in the first polymer coating. These tensile stresses, in turn, can cause the first polymer coating to fracture, or delaminate, or both, thereby compromising the strength and moisture resistance of the fiber. In addition, in the case of telecommunications fibers and stress-sensitive fibers such as polarization maintaining (PM) and polarizing (PZ) fibers, these tensile stresses can manifest themselves as microbending losses or other effects on the optical signal, thereby degrading the overall performance of the fiber.
In a first aspect, the invention features a method for coating an optical fiber that includes: (a) applying a photopolymerizable composition to an optical fiber having a surface coated with a first polymer coating; and (b) exposing the photopolymerizable composition to a source of actinic radiation to form a second polymer coating under conditions which inhibit the production of thermally induced tensile stresses in the first polymer coating.
In preferred embodiments, the fiber is cooled prior to application of the photopolymerizable composition. Preferably, this is accomplished by exposing the fiber to a chilled stream of gas (e.g., an inert gas such as helium).
Inhibiting the production of thermally induced tensile stresses in the first polymer coating during exposure may be accomplished in several ways. For example, the fiber may be cooled with a chilled stream of gas such as helium during exposure. Another protocol involves providing the source of actinic radiation with a dichroic reflector that transmits infrared radiation generated by the radiation source away from the fiber. Yet another useful protocol includes placing a water-cooled jacket concentrically about the fiber. The surface of the jacket may be further provided with an infrared radiation-absorbing coating. In another embodiment, a tube (e.g., a quartz tube) having a surface coated with an infrared radiation-absorbing coating is disposed concentrically about the fiber.
Each of these protocols may be used alone, or in combination with any, or all, of the others.
The actinic radiation preferably is ultraviolet radiation. The first polymer coating preferably includes an acrylate-functional silicone polymer, while the photopolymerizable composition preferably includes a photopolymerizable acrylate-functional epoxy or acrylate-functional urethane composition.
In a second aspect, the invention features a method for coating an optical fiber featuring a surface coated with a first polymer coating where the fiber is essentially free of a hermetic carbon coating underlying the first polymer coating. The method includes (a) cooling the fiber (e.g., by exposing the fiber to a chilled stream of gas such as helium gas); (b) applying a photopolymerizable composition to the first polymer coating; and (c) exposing the photopolymerizable composition to a source of actinic radiation to form a second polymer coating. Preferably, the method further includes inhibiting the production of thermally induced tensile stresses during exposure according to the procedures described above.
The invention provides optical fibers having multiple polymer coatings in which the production of tensile stresses within an individual polymer coating is minimized. The fibers exhibit good moisture resistance and mechanical properties, and resist delamination. The ability to minimize tensile stresses, and thus the defects associated with such stresses, makes the fibers particularly useful in defect-sensitive applications such as interferometric fiber optic gyroscopes.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims.